Tubing safety valves are used to close the tubing string in a well to control flow. Typically these valves have a disc that rotates 90 degrees against a seat that surrounds the flow path to close the valve. The disc, also known as a flapper, is forced open by a tube, known as a flow tube, which is hydraulically operated with pressure delivered from the surface to the housing of the valve via a control line that runs along the side of the tubing string. Control line pressure usually pushes the flow tube down against the flapper and the flapper gets behind the flow tube as it swings 90 degrees away from its seat. The control line pressure acts on an annular piston or one or more rod pistons that are operably connected to the flow tube. Downward movement of the flow tube is resisted by a closure spring so that in the normally open position for the flapper, the spring is compressed when the flapper is behind the downwardly displaced flow tube. As long as control line pressure is maintained the flow tube doesn't move and the valve stays open with the flapper behind the flow tube. When control line pressure is removed or otherwise lost, the closure spring has to be sized to overcome the hydrostatic pressure in the control line or annulus even when there is no applied pressure in the control line so that the flow tube can be biased up and a pivot spring on the flapper pivot connection can rotate the flapper 90 degrees to its seat to close the valve.
In very deep applications, the hydrostatic pressure in the control line can be very high and that requires a fairly large closure spring. Alternatives that have been used have their own drawbacks. For example a two line control system can be used so that hydrostatic pressure from the control lines cancels out on the annular or rod piston that has to move to actuate the flow tube. Sometimes there are space limitations and there is the time required to run another line and the added risks of damage to the control lines during installation. Another alternative is to provide a pre-charged housing chamber with enough gas pressure to offset the hydrostatic pressure in the control line so the closure spring only needs to overcome valve friction and the weight of the flow tube so that the valve can close on loss of control line pressure. However, providing pressurized chambers that can tolerate fairly high pressures depending of the operating depth of the valve make the valve more expensive to build and usually require many more seals and an intricate system of passages that can pose a greater risk of leakage that can undermine the operation of the valve.
Downhole safety valves that are the flapper type are shown in U.S. Pat. No. 6,957,703 and ball type downhole valve that apply an eccentric force to the ball to turn it 90 degrees between the open and the closed position are shown in U.S. Pat. No. 4,293,038.
In the realm of tools that are surface operated with control line pressure the present invention seeks to reduce or even eliminate the need for a closure spring or springs by taking advantage of available tubing weight. Usually there is a support further downhole from the downhole tool in the form of a packer or anchor, for example. In such cases the control line pressure can be used to lift at least a portion of the string above the valve sleeve housing where the housing is built to tolerate relative movement between components. An expansion joint can also be optionally used above the tool so that only the requisite weight of tubing above the valve housing is lifted. Alternatively in wells that experience a large range of temperatures, the thermal effects of temperature change can be addressed with such a bellows or expansion joint. Upon failure of control line pressure for any reason, the weight of the tubing can come into play and operate the downhole tool. The applications to the type of downhole tool are varied and those skilled in the art will more fully appreciate the scope of the invention from a review of the description of the preferred embodiment and the associated drawings that appear below while recognizing that the full scope of the invention is found in the claims below.